Low-temperature refrigerating system



Aug. 23,, 1949. w. E. DODSON LOW-TEMPERATURE REFRIGERATING SYSTEM Filed Aug. 5, 1947 Inventor. Waane EDodson. 51 1.724%

His. Atatornqs.

Patented Aug. 23, 1949 SYSTEM Wayne E. Dodson, Caldwell, N. J., assignor to General Electric Company, a corporation of New York Application August 5, 1947, Serial No. 766,386

7 Claims. (Cl. 62-116) My invention relates to refrigerating systems for operation at low temperatures and particularly to such systems for cooling air in cold storage compartments. test rooms, and the like.

Cold storage rooms are commonly cooled by at the higher temperatures employed, for example, in the temporary preservation of foods. Accordingly, its is an object of my invention to provide a low temperature refrigerating system wherein the major portion of the heat load may be removed at relatively high coefficients of performance of the refrigerating machine.

It is another object of my invention to provide a low temperature refrigerating machine including an improved and more eflicient arrangement for maintaining low temperatures within a compartment to be cooled.

It is a further object of my invention to provide an improved refrigerating system for operation at low temperatures and which shall make possible the application of a smaller refrigerating machine for carrying a predetermined heat load.

Further objects and advantages of my invention will become apparent as the following description proceeds, and the features of novelty which characterized my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of my invention, reference may be had to the accompanying drawing in which Fig. 1 represents diagrammatically a low temperature refrigerating system embodying my invention; Fig. 2 is a diagrammatic view of a wall of the system of Fig. 1 with a graph illustrating a characteristic of operation of the system; Fig. 3 represents a modified fonn of the invention; and Fig. 4 represents a further modification of the invention.

Referring now to the drawing, the refrigerating system illustrated in Fig. 1 comprises a thermally insulated cabinet or room 5, a portion only of which has been illustrated, and which is provided with inner and outer retaining walls r a 2 2 and 3 which are spaced apart and accommodate a body of insulation 4 therebetween. Within the cabinet there is provided a compartment 5 which is cooled by an evaporator 8, the air in the compartment being circulated over the evaporator by operation of a, fan I driven by an electric motor 8. The use of motor 8 and fan I is optional as it will be understood that circulation by gravity may be used. Liquid refrigerant is supplied to the evaporator 6 from a condensing unit 9, the refrigerant flowing from a liquid line In through a branch liquid line H to the evaporator under control of a thermostatic expansion valve I2. The valve is provided with a feeler bulb or temperature control element l3 connected in heat exchange relation with a suction line I through which vaporized refrigerant is withdrawn from the evaporator and returned to the condensing unit. The condensing unit may be controlled in any suitable manner in accordance with wellknown practice to maintain the air in the compartment 5 within a suitable predetermined range of temperatures, for example a temperature in the neighborhood of sixty degrees below' 4 zero Fahrenheit. When a refrigerated compartment is operated at such low temperatures, there is a very substantial load due to the leakage of heat from the air outside the cabinet through the insulation to the evaporator; and, if the evaporator within the compartment is required to remove this heat at-the low temperature of the compartment, the refrigerating machine must operate at a relatively low coeflicient of performance. In order to reduce the load due to leakage through the cabinet walls, a second evaporator is arranged within the insulation 4. This evaporator comprises a refrigerant conduit having a plurality of turns or runs l5 extending back and forth through the insulation in planes substantially parallel to the walls 2 and 3 and distribuated over a major portion of the thermally insulated walls of the compartment. It will be understood that the location and thickness of the insulated walls are determined by the arrangement of the refrigerated compartment 5 with respect to other adjacent refrigerated areas, and that the amount of insulation required will vary depending upon the normal ambient temperatures to be encountered at various portions of the outer wall 3. The evaporator I5 is connected to receive liquid refrigerant from the liquid line H) through a branch line is under control of a thermostatic expansion valve IT. The valve i7 is provided with a temperature responsive element iii in heat exchange relation with the discharge or suction line of the evaporator I5 as indicated at I9. By arranging the evaporator I5 in this manner it is possible to abstract 8. large quantity of heat from the insulated wal1 so that a major portion of the temperature drop between the outside air andthe air in the cabinet takes place between the outside wall 3 and the evaporator l5. Since the heat-thus removed is removed at a relatively high temperature, the coeflicient of performance of the refrigerating machine is relatively high. Obviously, the heat removed by evaporator l5 reduces the amount to be removed through the evaporator 8 at the necessarily lower coeflicient of performance.

The evaporator 6 is operated at a relatively low pressure in order to abstract heat at a low temperature from the air in the compartment 5, and the evaporator I5 is operated at a substantially higher pressure for operation at the higher temperature. In order to maintain the two different temperatures in the evaporators 6 and I5, the condensin unit 9 is constructed as a twostage or multi-stage compressor. As illustrated, the condensing unit includes two high pressure cylinders 20 and H and a low pressure cylinder 22 all provided with suitable pistons (not shown). Refrigerant compressed in the high pressure cylinders is delivered through a discharge line 23 to a condenser 24 where it is cooled and liquefied and then flows to a liquid receiver 25 to which the liquid line I is connected. In order to operate the evaporator 6 at a low pressure, the cylinders 2| and 22 are connected in tandem as a twostage unit, refrigerant from the evaporator 6 being delivered to the cylinder 22 from the line I4 and compressed and then discharged through an interstage connection 26 to the intake of the cylinder 2|. The evaporator I is connected through the suction line I9 to the high pressure cylinder 20. The pistons of the high pressure cylinders and of the low pressure cylinder may be actuated by the same motor (not shown) or by separate motors.

The operation of the refrigeratin system shown in Fig. 1 maintains the required temperature gradient or drop between the ambient air outside the wall 3 and the air within the compartment 5. By way of example, Fig. 2 illustrates diagrammatically the manner in which the temperature gradient is maintained. Assuming by way of illustration that it is desired to maintain a temperature within the compartment 5 of one hundred degrees below zero Fahrenheit and that the temperature of the outside air is one hundred degrees above zero Fahrenheit, then the total temperature drop is two hundred degrees. In a wall having a thickness of the order of six inches and with suitable thermal insulation, the evapo--- rator I5 may be arranged in a plane about two inches from the wall 3 and about four inches from the wall 2, these distances being indicated by the letters a and b respectively. If the refrigerating system is operated so that the evaporator I5 maintains a temperature of fifty degrees below zero Fahrenheit, then about three-fourths of the temperature gradient will occur between the wall 3 and the evaporator, as indicated at 21, and the remaining one-fourth between the evaporator and the wall 2, as indicated at 28. Under these conditions of operation, assuming that the insulation has a coefi'icient of conductivity of 0.3 B. t. u./hr./sq. ft. of surface/degree Fahrenheit, then for a one hundred square foot area of the wall a total of 2250 B. t. u./hr. will enter the wall 3, 1875 B. t. u./hr. will be absorbed 4 by the evaporator I5, and 375 B. t. u./hr. by t evaporator 6. On the other hand, should the evaporator I5 be cut out of operation, then the evaporator 6 would be required to carry a load of approximately 1000 B. t. u./hr., this being the total leakage in a sq. ft. area when a temperature of one hundred degrees below zero Fahrenheit is maintained in the compartment 5 with an ambient temperature of one hundred degrees Fahrenheit. Although in this latter case only 1000 B. t. u./hr. need be removed by the refrigerating machine, this heat must be removed at a low coeflicient of performance and a relatively large refrigerating machine will be required. By employing the evaporator I5 the total required capacity of the refrigerating machine is substantially less.

Under some conditionsof operation, particularly during pull-down when the temperature within the compartment 5 is high, it may be desirable to operate the evaporator I5 first in order to remove the stored heat from the walls before the temperature of the compartment B'is reduced materially. After the stored heat has been removed in this manner to an extent sufiicient to bring the temperature at the evaporator I5 down to the required value, the evaporator 6 may be operated to lower the temperature within the compartment 5 quickly to its required value. This type of operation is indicated in Fig. 2 by the dotted line 21a, which represents the gradient between the high temperature within the compartment 5 and the low temperature of the evaporator I5 at the time when the evaporator 6 is to be started. A control arrangement for effecting this type of operation is disclosed and claimed in my copending application Serial No. 766,387, filed concurrently herewith and assigned to the same assignee as the present invention.

In the system of Fig. 3, only the arrangement of the condensing unit has been illustrated, as the remaining portions are identical with that of Fig. 1. Corresponding elements of Fig. 3 have been designated by the same numerals as in Fig. 1. The condensing unit of Fig. 3 is a two-stage compressor having a high pressure cylinder 29 and a low pressure cylinder 30. These cylinders are provided with pistons (not shown) driven by the same motor (not shown) or by separate motors as desired. Refrigerant withdrawn from the evaporator 6 through the suction line I 4 is compressed in the cylinder 30 and is delivered through a discharge connection 3| to the suction line I9 of the evaporator I5 and is thus delivered to the intake of the cylinder 29. The discharge connection 3| and the end portion of the conduit I9 thus provide an interstage connection between the cylinders 29 and 30. The refrigerant withdrawn from the evaporator I5 and that delivered from the compressor 30 are compressed in the cylinder 29 and delivered through a dis-' charge line 32 to the condenser 24 where the refrigerant is liquefied and fiows to the liquid receiver 25 and thence through the liquid line I0 back to the evaporator. The operation of the system of Fig. 3 to maintain the required temperatures and to secure the desired relatively high overall coeflicient of performance is essentially the same as that of Fig. l, the evaporator, I5, however, being operated at interstage pressure'instead of at a pressure determined by operation of a separate compressor, as in Fig. 1.

The modification of Fig. 4 is similar to that of Fig. 1 and corresponding parts have been designated by the same numerals. The system of Fig. 4 differs from that of Fig. .1 in that the compressing apparatus comprises two separate refrigerating machines 33 and 34 connected to the evaporators 6 and I5, respectively. The refrigerating machine .33 comprises a compressor 35, a condenser 36 and a liquid receiver 31. Vaporized refrigerant withdrawn from the evaporator 6 through the suction line 14 is delivered to the compress-or 35 where it is compressed and discharged to the condenser 36 to be liquefied, the liquid refrigerant collecting in the receiver 31 and flowing directly to the liquid line H of the evaporator. The compressor 35 is operated to I maintain a predetermined low pressure and may be controlled in accordance with the temperature of the air in the compartment in the usual manner. Refrigerant vaporized in the evaporator i 5 is delivered through the suction line IE to a' compressor 38 of the unit 34 and is liquefied in a condenser 39 fromwhich it flows to a liquid receiver 40 and returns to the evaporator through the liquid line I6. The compressor 38 operates at a higher pressure than the compressor 35 and maintains the evaporator l5 at the desired higher temperature. The compressor 38 may be controlled in any suitable manner, for example, in accordance with suction line pressure or the temperature of the evaporator IS. The condensing unit 34 thus operates at a higher coefficient performance than the condensing unit 33 and, as indicated by the sample characteristics of Fig. 2 which are applicable to all three illustrated embodiments of the invention. The machine 34 may be arranged to carry the major portion of the heatload so that the over-all coefficient of performance of the refrigerating apparatus of Fig. 4 is thereby raised substantially over that of the coefficient of performance of the condensing unit 33 alone.

While I have illustrated my invention in connection with particular types of refrigerating machines, other applications will readily occur to those skilled in the art.- For example, although the system illustrated employ evaporators directly for cooling the walls and interior of the storage compartment, it is obvious that the evaporators may be employed to cool brine or other suitable secondary refrigerant which is then circulated through cooling units arranged in the walls and in the storage compartment. I do not,

therefore, desire my invention to be limited to the specific details illustrated and described, and I intend by the appended claims to cover all modifications within the spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a low. temperature refrigerating system, means including a thermally insulated wall for providing a chamber to be cooled, a first cooling element arranged to abstract heat from the medium within said chamber, a second cooling element arranged within said wall, means including a refrigerant compressing apparatus having a high pressure compressor and a low pressure compressor for supplying cooling fluid to said elements, and means for operating said low pressure v Y perature.

2. In a low temperature refrigerating system, means including a thermally insulated wall for providing a chamber to be cooled, a first cooling element arranged within said chamber to cool the air therein, a second cooling element comprising a plurality of runs of tubing distributed throughout a major area of said wall and arranged substantially in a plane parallel .to the surfaces of said wall, means including a refrigerant compressing apparatus having a high pressure compressor and a low pressure compressor for supplying cooling fluid to said elements, and means for operating said low pressure compressor to abstractheat from said first element at a predetermined low temperature and said second compressor to abstract heat from said second element at a predetermined higher temperature.

3. In a low temperature refrigerating system, means including a thermally insulated wall for providing a chamber to be cooled, a first cooling element arranged within said chamber to cool the air therein, a second cooling element comprising a plurality of runs of tubing distributed throughout a major area of said wall and arranged substantially in a plane parallel to the surfaces of said wall, means including a refrigerant compressing apparatus having a high pressure compressor and a low pressure compressor for supplying cooling fluid to said elements, and means for operating said low pressure compressor to abstract heat from said first element at a predetermined low temperature and said high pressure compressor to abstract heat from said second element at a predetermined higher temperature, said plane in which said second element is arranged being located within the thermal insulation between said inner and outer walls in a position such that the major portion of the temperature drop at normal room temperatures from the outer wall to the inner wall occurs between the outer wall and said plane.

4. In a low temperature refrigerating system, means including a thermally insulated wall for providing a chamber to be cooled. a first cooling element arranged to abstract heat from the medium within said chamber, a second cooling element arranged within said wall. means including a two-stage refrigerant compressing apparatus for supplying cooling fluid to said elements, and means for operating the low pressure stage of said apparatus to abstract heat from said first element at a predetermined low temperature and the high pressure stage of said apparatus to abstract heat from said second element at a predetermined higher temperature.

5. In a low temperature refrigerating system, means including a thermally insulated wall for providing a chamber to be cooled, a first cooling element arranged to abstract heat from the medium wtihin said chamber, a second cooling element arrangedwithin said wall, means including a two-stage refrigerant compressor comprising a high pressure cylinder and a low pressure cyl inder for supplying cooling fluid to said elements, means for utilizing said low pressure cylinder to abstract heat from said first element at a predetermined low temperature, and means including an interstage connection between said cylinders for utilizing said high pressure cylinder to abstract heat from said second element at a predtermined higher temperature.

' 6. In a low temperaturerefrigerating system, means including a thermally insulated wall for providing a chamber to be cooled, a. first cooling element arranged to abstract heat from the medium within said chamber. a second cooling element arranged within said wall, means comprising a compressing apparatus including two means including a thermally insulated wall for providing a chamber to be cooled, a firstevaporator arranged in said chamber for cooling the air therein, a second refrigerant evaporator arranged within the insulation in said wall and extending throughout a major area of said wall,

a two-stage refrigerant compressor having a low pressure cylinder and'a high pressure cylinder, means including a condenser connected to receive refrigerant from said compressor for liquefying refrigerant and supplying liquid refrigerant to said evaporators, means including a connection between said first evaporator and said low pressure cylinder for operating said first evaporator at a predetermined low temperature, and means including aconnection between said second evaporator and said high pressure cylinder for operating said second evaporator at a higher predetermined temperature.

WAYNE E. DODSON.

No references cited. 

